Innervates all effector organs and
tissues except for skeletal muscles. It is autonomic because
it functions subconsciously and involuntarily.

Dual Innervation

There are two branches or
divisions of the autonomic nervous system (ANS): the sympathetic and
parasympathetic. Both branches innervate most organs in an
arrangement called dual innervation. The parasympathetic division
is most active during rest and stimulates digestive activities. The sympathetic
division is most active during times of excitement and physical
activity.
Animation Comparing Sympathetic and Parasympathetic Nervous Systems

The two divisions of the ANS
work together to finely control the functions of the various organs
so that they operate appropriately in different situations. The two
systems often perform this control by working at cross purposes. So, for
example, to precisely control the heart rate the sympathetic
division will increase it while the parasympathetic decreases it.

Anatomy of the ANS

The commands of the ANS leave the
central nervous system and go to effector organs by means of
two efferent neurons arranged in
series. The first neuron (preganglionic
neuron) synapses with the second neuron (postganglionic
neuron) at an autonomic ganglion.

Preganglionic neurons of the sympathetic
nervous system originate in the region of gray matter in the
thoracic and upper lumbar region called the lateral horn. Pre- and postganglionic neurons are arranged in three
patterns:

1. The preganglionic neuron leaves through the
ventral root. After the ventral and dorsal roots fuse to form
the spinal nerve the preganglionic neuron goes to sympathetic
ganglia that are connected to one another and run parallel to
the spinal column on either side. The
chain of ganglia is the sympathetic chain or sympathetic
trunk.

The preganglionic neuron is
myelinated and the
axons of these form the white ramus (pl. rami) or "white
branch" as they connect to the ganglia. The
postganglionic neurons are unmyelinated and leave the ganglion
as the gray ramus which rejoins the spinal nerves.

Because the axons of the
preganglionic neurons may
branch and travel up and down the
sympathetic chain, a singlepreganglionicneuron
can synapse with manypostganglionic neurons up
and down the sympathetic chain. Hence, the effects of
stimulation are widespread.

2. A group of long preganglionic neurons
innervate modified postganglionic cells in the adrenal medulla
called chromaffin cells. These cells release
epinephrine (80%), norepinephrine (20%) and a small amount of
dopamine into the bloodstream in response to stimulation. The
effects of these hormones spread by the bloodstream are widespread.

3. The preganglionic neurons synapse with
postganglionic neurons in collateral ganglia. The
preganglionic neurons leave the spinal nerve through the white
rami but do not synapse with neurons in the sympathetic chain
but continue to the collateral ganglia.

The postganglionic neurons
originating from the collateral ganglia travel to specific
effector organs. For example, postganglionic neurons
originating from the celiac ganglion innervate some of
the digestive organs. Hence, the effects of stimulation by
this route are more localized and discrete.

The preganglionic neurons
originate in the brain stem or sacral spinal cord and
are relatively long. The preganglionic neurons synapse with
postganglionic neurons in ganglia near the effector organ or in
the wall of the effector organ.

Cranial nerve nuclei are in
the brainstem and travel with cranial nerves III, VII, IX and
X. The X is called the vagus and innervates much of the viscera
including lung, heart, stomach, small intestines and liver.

The parasympathetic neurons
that originate from the sacral spinal cord join to form distinct
pelvic nerves.

Autonomic Neurotransmitters

The peripheral nervous system uses two
neurotransmitters:

1. Acetylcholine is the most common. Neurons that release it are called cholinergic. Cholinergic
neurons include all preganglionic neurons of the autonomic
nervous system, postganglionic neurons of the parasympathetic
nervous system and some
postganglionic neurons (sweat glands) of the sympathetic
nervous system.

2. Norepinephrine is the
other neurotransmitter and is released by neurons called adrenergic.
Almost all sympathetic postganglionic neurons are
adrenergic.

Cholinergic Receptors

There are two classes:

1. Nicotinic receptors are found on cell
bodies and dendrites of sympathetic and parasympathetic
neurons, on chromaffin cells of the adrenal
medulla and on skeletal muscle cells. These receptors are
associated with cation channels that allow both potassium and sodium
ions to pass through. These receptors are associated with depolarization
of the postsynaptic cells.

2. Muscarinic receptors are found on
effector organs of the parasympathetic nervous system. These
receptors are coupled to G proteins and may either
be
inhibitory or excitatory. Effector organs acted upon include
heart and smooth muscles of the pupil and digestive tract.

Adrenergic Receptors (Table 11.1)

There are two classes of receptors
aand b
and each of these is divided into subclasses (a1and a2) and (b1,b2

and b3

). These receptors are coupled to G proteins that
activate or inhibit second messenger systems.

have much more affinity for epinephrine and produce an
inhibitory response. (N. B. Whether a G protein is
stimulatory or inhibitory does not predict the cellular
response. The cellular response, such as muscle contraction or
glandular secretion, may be either promoted or inhibited.)

Autonomic Neuroeffector Junctions

Synapses between an efferent neuron and
its effector organ is a neuroeffector junction. In contrast
to typical axon terminals, neurotransmitters are released at
swellings along the axon called varicosities.
When action potentials reach these varicosities they are propagated
by voltage-gated Na+ and K+ channels but in
addition have voltage-gated Ca++ channels.

Action potentials that reach the
varicosities open up Ca++ channels and Ca++
rushes into the cell triggering the release of neurotransmitter.
Transmitter is released at all the varicosities so its release is
more widespread than at typical axon terminals.

Regulation of Autonomic Functions

The sympathetic and
parasympathetic divisions of the autonomic nervous system often work
in opposition in order to maintain homeostasis. Hence the competing
influences of the sympathetic and parasympathetic divisions need to
be balanced or regulated to achieve homeostasis.

Most changes in the organ activity are
controlled by visceral reflexes.
These reflexes include the pupillary light reflex, accommodation,
vomiting reflex, swallowing reflex, urination, defecation, erection
and ejaculation. Higher centers that control autonomic function
include the hypothalamus, pons and medulla oblongata.

The hypothalamus exerts
an overriding influence on autonomic functions. It initiates
the flight-or-fight response by activating the sympathetic branch
which has immediate widespread effects. The hypothalamus contains
centers that regulate body temperature, food intake and water
balance.

The hypothalamus, in turn,
receives input from the cerebral cortex and the limbic
system which is concerned with the experience of emotions.

Anatomy of Somatic Nervous System

Somatic motor neurons originate in the
ventral horn of the spinal cord. A single motor neuron innervates
many skeletal muscle fibers. A motor neuron plus all the fibers
innervated is a motor unit.

Neuromuscular Junction

The neuromuscular junction is the
region where the motor neuron synapses with a skeletal motor
fiber. The axon
terminals of the motor neuron are called terminal boutons. Acetylcholine
is stored and released here. The plasma membrane opposite the
terminal bouton is called the motor end plate. The motor end
plate is invaginated and contains nicotinic cholinergic receptors. Acetylcholinesterase
is found in the synaptic cleft and terminates the excitatory
signal.

When an action potential arrives at the
terminal bouton the depolarization causes Ca++ channels
to open and Ca++ enters the cell. This triggers the
release of acetylcholine by exocytosis. Acetylcholine binds to
nicotinic cholinergic receptors at the motor end-plate opening
cation channels. Na+ flows into the muscle cell and
causes an end plate potential that is sufficient to trigger
an action potential. The action potential leads to contraction of
the fiber.